Phylum Platyhelminthes

Characteristics
Members of phylum Platyhelminthes have a flattened, bilaterally symmetrical body. They have three embryonic layers (triploblastic): the endoderm, the mesoderm, and the ectoderm. Flat worms have no other body cavity than the digestive cavity (acoelomates).

Classification
Members of the class Turbellaria have bodies covered with cilia and are free living. Members of the class Trematoda are parasites with a thick cuticle instead of cilia. Members of the class Cestoda are also parasites with a thick cuticle.

Protection
Parasitic flat worms have a thick cuticle for protection. Free living flat worms rely on camouflage.

Movement
Turbellians use cilia to move while parasitic worms rely on the host's circulation.

Feeding & Digestion
Flat worms have one digestive opening that branches to all parts of the body. Turbellarians have a muscular pharynx to up food. Parasitic classes rely on the host for digestion.

Circulation
Circulation occurs through diffusion.

Excretion
Excretion occurs through flame cells.

Respiration
Respiration occurs through diffusion.

Nervous System
Platyhelminthes have a simple brain called a ganglion and a ventral nerve cord.